Multivibrator



June 20, 1961 T. B. BARTLETT 2,989,704

MULTIVIBRATOR Filed April 15, 1959 INVENTOR THOMAS B. BARTLETT BY @wa 4142;

ATTORNEY United States Patent 2,989,704 MULTIVIBRATOR Thomas B. Bartlett, Davenport, Iowa, assignor, by mesne assignments, to The Gamewell Company, Newton, Mass., a corporation of Delaware Filed Apr. 15, 1959, Ser. No. 806,465 6 Claims. (Cl. 328200) This invention relates to multivibrators and in particular to bistable multivibrators for divide-by-two circuit applications not requiring extremely short trigger pulses.

The low repetition rate bistable multivibrator disclosed herein has the advantage of simplicity in that two neon glow tubes are employed for timing the transfer of the circuit in place of two capacitors and two resistors. The glow tubes are less expensive than the capacitors and resistors and require less space. They also give a visual indication of which phase the circuit is in. The output amplitude of the circuit is more stable to supply voltage fluctuations, and biasing is not quite as critical. The circircuit requires, however, that the plate load resistances be adjusted relative to one another to compensate for variations in tube halves and for variations in the gas diodes.

One important use for low repetition rate multivibrators is in industrial computers in which the input signals arrive at a low rate. The input signals may be developed during the time an operation on a unit of production is being completed. Since an operation requires a finite time to complete, input signals are initiated at finite intervals.

Another use of the invention is in a vehicular trafiic counting device. Such a device is described in United States patent application Ser. No. 738,327, filed May 28, 1958, and entitled Trafiic Lane Control. A traffic volume computer is fed impulses froma bistable multivibrator which receives impulses from a group of blocking oscillators. Each blocking oscillator is triggered from a pressure pad mounted in a traffic lane. The pad is depressed first by the front wheels of each vehicle and then by the rear wheels. The two pulses are reduced to a single pulse in the subject invention after being given a uniform potential and duration by the blocking oscillators.

The rate of arrival of impulses originated by passing vehicles is low and is well within the repetition rate of the subject invention.

The principal object of the invention is to provide a low speed, simple, inexpensive divide-by-two circuit.

Another object is to provide a bistable multivibrator utilizing gas filled diodes in place of the usual feed back resistors and capacitors.

Another object is to provide a multivibrator having inherent means for indicating which phase is conducting.

Another object is to provide a multivibrator whose output voltage is stable and whose Wave form is square.

The invention will be described with reference to the figure shown on the accompanying drawing. The output pulses from a prior stage are applied to input terminal 1 and are impressed on the grids 2, 3 of both halves 4, 5 of double triode vacuum tube 6 through silicon dioxide rectifiers 7, 8. The cathodes 10, 11 of both tube halves are connected together and to ground through resistor 12. B+ potential is applied to the plates 13, 14 through potentiometer portions 15, 16, respectively. B+ voltage is applied to the center tap 17 and the tap is adjusted so that difierent potentials may be applied to the two plates 13, 14 to compensate for differences in tube characteristics.

The grid 2 of tube half 4 is connected to the plate 14 of tube half 5 through neon glow tube 18. Grid 3 is likewise connected to plate 13 through glow tube 19. The output signal is available at terminal 20 through coupling 2,989,704 Patented June 20, 1961 ice capacitor 21. Resistors 22, 23 are grid leak resistors connected to grids 2, 3, respectively.

In operation, the bistable multivibrator receives pulses at terminal 1, and for each two impulses received puts out one impulse at terminal 20. One half of the double triode is normally conducting. Assume for example that tube half 4 is conducting and that no signal is present at terminal 1. The path of electron flow is from ground, through cathode resistor 12, from the cathode 10 to plate 13 of tube half 4, and through portion 15 of the plate resistor, and through center tap 17 to B+.

While tube half 4 is conducting there is very little voltage drop across it. Impedances 15 and 12 act as a potential divider, their values being chosen so that the po tential appearing at junction 24 is less than that required for the ionization of neon glow tube 19. Glow tube 18 is ionized, however, because tube half 5 is not conducting and approximately full B+ potential appears at junction 25. The path of electron flow through glow tube 18 includes ground, grid leak resistor 22, grid 2, glow tube 18, potentiometer portion 16, center tap 17, and B+. The grid 2 of tube half 4 is at positive potential causing tube half 4 to conduct. The positive potential is not fed back to input terminal 1 because it is blocked by rectifier 7.

The positive potential at junction 25 causes a capacitor 21 to charge and draw current from the output terminal 20 through the load. The grid 3 of tube half 5 is. at zero potential because glow tube 19 is not conducting. Thus, tube half 5 is made non-conducting.

Assume now that a negative pulse is applied to input terminal 1, resulting, for example, from the actuation of a detector by the front wheels of a passing vehicle. The negative pulse is passed to both grids 2, 3 through rectifiers 7, 8. Making grid 3 negative has no effect on tube half 5 because it is already non-conducting.

When grid 2 is made negative, however, tube half 4 is cut off. The potential appearing at junction 24 rises almost to B+ value and causes neon glow tube 19 to ionize almost immediately. Grid 3 is thereby raised in potential. The path of electron flow includes grid leak resistor 23, grid 3, glow tube :19, potentiometer portion 15, tap 17, and B+.

Conduction of tube half 5 results as the potential of grid 3 increases. The path of electron flow includes ground, cathode resistor 12, cathode 11, plate 14, potentiometer portion 16, tap 117, and B+. As conduction through tube half 5 increases, the positive potential appearing at junction 25 decreases because the drop across potentiometer portion 16 increases. Neon glow tube 18 does not reionize.

The positive potential appearing at junction 24 is almost equal to the potential of B+ because tube half 4 is not conducting and no potential drop occurs across potentiometer portion 15. Neon glow tube 19 ionizes almost immediately and impresses almost all of this positive potential on grid 3, aiding tube half 5 to conduct more heavily. The values of the cathode resistor 12 and the plate resistors 15 and 16 are suflicient to prevent the tube halves 4, 5 from conducting too heavily and overheating.

The circuit is stable in this condition also. The reduced potential appearing at junction 25 causes the left side of capacitor 21 to become less positive. Capacitor 21 partially discharges through output terminal 20 and the load. Thus, the first negative pulse appearing at input terminal 1 results in a negative pulse at the output terminal 20. The second pulse at input terminal 1 does not result in a pulse at output terminal 20 and thus the circuit is a divideby-two circuit.

While tube half 5 is conducting and tube half 4 is not conducting, assume that the second negative impulse appears at terminal 1. The second impulse may result, for

example, when the rear wheels of a vehicle pass over the detector.

The negative pulse is applied to both grids 2, 3 through rectifiers 7, 8. Making the grid 2 negative has no effect on tube half 4 because it is already non-conducting. Making the grid 3 negative causes tube half 5 to stop conducting. The potential of junction 25 rises rapidly and becomes sufiiciently high to ionize glow tube 18 and cause it to conduct. Grid 2 is thus made more positive because the drop across glow tube 18 is decreased. Tube half 4 begins to conduct and the potential of junction 24 is reduced and glow tube 19 becomes deionized. Grid 3 becomes less positive and cuts off tube half 5. The potential of junction 25 rises almost to the value of B+ and this higher potential also appears on grid 2 because there is little drop through the ionized glow tube 18. Thus, tube half 4 and glow tube 18 are both conducting.

The positive potential appearing at junction 25 and at the left side of capacitor 21 causes capacitor 21 to recharge, drawing current from the output terminal 20 and the load.

The circuit is stable in each conducting condition but an output pulse results only when the conduction of the right side is increasing. Thus, for each two negative impulses received, only one negative impulse is transmitted.

Pulses arriving at input terminal 1 must be spaced at greater intervals than required for the transfer and retransfer of the multivibrator. If they arrive at too high a rate, some will be missed. For the applications noted above the circuit is well within its range.

For good results, relative values of the various components have been found from experience to be equal to the following:

Tube 6 5814-A Rectifiers 7, 8 HD-6l25 Cathode resistor 12 ohms 20,000 Potentiometer 15, 16 do 100,000 Diodes 18, 19 NE-Z Coupling capacitor 21 microfarad 0.001 Grid leak resistors 22, 23 ohms 100,000 3-}- potential volts +150 Input potential do -50 It is understood that the description and the figure are made by way of illustration and are not by way of limitation. The spirit of the invention is limited only by the terms of the appended claims.

I claim:

1. In a bistable multivibrator, a first tube having at least a cathode, an anode, and a control grid; an input terminal, a rectifier connected between said input terminal and said grid, a grid leak resistor connected between said grid and ground; a second tube having at least a cathode, an anode, and a control grid, a second rectifier connected between said input terminal and said second grid, a grid leak resistor connected between said second grid and ground; the cathodes of said first and second tubes connected together and to ground through a cathode resistor, a potentiometer having one lead connected to the plate of said first tube and another lead connected to the plate of said second tube and having an adjustable tap, a source of positive potential connected to said tap, a first neon glow tube connected between said first plate and said second grid and a second neon glow tube connected between said second plate and said first grid, an output terminal, and a coupling capacitor connected between said output terminal and said second plate.

2. A bistable multivibrator including a double triode vacuum tube having first and second cathodes, control grids, and plates, first and second neon glow tubes connected between said first grid and said second plate and between said second grid and said first plate, a source of positive potential, at potentiometer connected between said plates and having an adjustable tap connected to said source of potential, a cathode resistor, said cathodes connected together and to ground through said cathode resistor, a first grid leak resistor connected between said first grid and ground, a second grid leak resistor connected between said second grid and ground, an input terminal, a first rectifier connected between said input terminal and said first grid, at second rectifier connected between said input terminal and said second grid, an output terminal, and a coupling capacitor connected between one of said plates and said output terminal.

3. A bistable multivibrator comprising first and second vacuum tubes each having at least a cathode, a grid, and a plate; a cathode resistor, said cathodes connected together and to ground through said cathode resistor, a plate circuit including a source of potential and a potentiometer having a tap connected to said source, each end of said potentiometer connected to one of said platcs, a pair of grid leak resistors each connected between ground and one of said grids, a source of input pulses, a pair of rectifiers, each of said grids connected to said source of input pulses through one of said rectifiers, a two-element cross coupling circuit consisting of a first neon glow tube connecting the grid of said first tube to the plate of said second tube and of a second neon glow connecting the grid of said second tube to the plate of said first tube, and an output terminal and coupling capacitor connected to one of said plate circuits.

4. A divide-by-two bistable multivibrator circuit having two triode vacuum tubes, a regenerative circuit comprising a gaseous discharge tube connected between the anode of each triode and the control grid of the other triode, a self-limiting cathode circuit including the cathodes of both of said triodes connected together and through a common impedance to ground, an adjustable plate circuit connected between a source of plate potential and the plate of each triode, a signal input circuit comprising an input terminal and a pair of unidirectional impedance elements connected between the control grids of each triode and said input terminal, and a grid current limiting circuit comprising a pair of impedances connected between each of the control grids and ground.

5. In a divide-by-two bistable multivibrator circuit having two triode vacuum tubes, a regenerative circuit comprising solely a pair of gaseous discharge tubes each one connected between the anode of one of the triodes and the control grid of the other of the triodes.

6. In a divide-by-two multivibrator having a double triode vacuum tube, a regenerative circuit comprising a pair of neon glow tubes each one connected between the anode of each triode and the control grid of the other triode.

Waveforms by Chance et 211., published by McGraw- Hill, NY, 1949 ed., pages 164 to 166. 

